The concept of "flow" (a term coined by American psychologist Mihaly Csikszentmihalyi that means "a condition of heightened focus, productivity, and happiness that we all intuitively understand and hunger for") can be a bit nebulous, but it's interesting and something that can be applied to everyday work and life. Fast Company has a nice peice on Flow. From the article:

In the flow state, Csikszentmihalyi found, people engage so completely in what they are doing that they lose track of time. Hours pass in minutes. All sense of self recedes. At the same time, they are pushing beyond their limits and developing new abilities. Indeed, the best moments usually occur when a person's body or mind is stretched to capacity. People emerge from each flow experience more complex, Csikszentmihalyi found.